Hydrogen Fires

Jennifer X WenFire and Explosion Research Group

Faculty of Engineering, Kingston UniversityLondon, UK

Outline (Lecture 1)

• Basic characteristics of hydrogen fires

• Some fundamentals of fires

• Hydrogen jet fires– Flame Length– Lift-off heights– Flame radiation– Horizontally oriented high pressure hydrogen jet flames – Ignition of Hydrogen Leaks

Basic characteristics of hydrogen fires

• Hydrogen fire burns quickly

• Hydrogen flames have low radiant heat

• Hydrogen fires are almost invisible

Basic characteristics of hydrogen fires

Table 3 Combustion time for 121 litres of fuel (reproduced from [1])

Fuel Combustion time

Liquid hydrogen (LH2) 27 sec

Propane 4 min

Petrol 5 min

Jet fuel (JP-4) 7 min

Comparison of fire from a leak in a gasoline engine car and the same kind of leak from a hydrogen car. Gasoline car to the right and hydrogen car to the left (reproduced from [2])

Hydrogen Fire HazardsTable 11 Frequency of occurrence of ignition sources (reproduced from [47])

Ignition source Hydrogen incidents Non-hydrogen incidents

Number % Number %

Arson 0 0 37 2.6

Collison 2 2.5 121 8.4

Flame 3 3.7 113 7.9

Hot surface 2 2.5 56 3.9

Electric 2 2.5 114 7.9

Friction spark 2 2.5 33 2.3

Not identified 70 86.3 942 64.5

Non-ignition 0 0 21 1.5

Total 81 100 1437 100

Some fundamentals of fires

• Premixed combustion • Non-premixed combustion

Partially premixed combustion

Fireballs, jet, pool and flash fires

• A fireball results from the burning of fuel-air cloud

• Jet fires result from the combustion of a fuel released from a pressurized system or storage vessel.

• Pool fires are the result of surface burning of flammable or combustible liquid

• A flash fire is the non-explosive combustion of a vapour cloud resulting from a release of flammable or combustible material into the open air.

Jet Fires

Fireball

Liquid Hydrogen and Oxygen Fireball230 kg (500 lb) Test

Hydrogen jet fires - Flame Length

• Delichatsios’s correlation– Nondimensional Froude number

– Nondimensional flame length

Frf =

ue fs3/ 2

(ρe /ρ∞)1/ 4[∆Tf

T∞gd j]

1/ 2

5/12

5/2

)07.01(5.13

*f

f

FrFr

L+

=

*)/(

* 2/1 dLf

dLf

L s

ej

s ==∞ρρ

– In the buoyancy-dominated regime,

for Frf < 5

– In the momentum-dominated regime,

for Frf < 5

23* =L

Hydrogen jet fires - Flame Length

• Delichatsios’ correlation • With Sandia vertical hydrogen flame data

Hydrogen jet fires - Flame Width

Variation of visible flame width with Froude number. Data Ratio of maximum flame width to flame length determined from visible flame emission (reproduced from [20).

Hydrogen jet fires – Lift-off height

• Kalghatgi’s correlation from subsonic jets to those from under-expanded sonic jets: 5.1

max

max⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=⎟⎟

⎠

⎞⎜⎜⎝

⎛

∞ρρ

µρ e

L

e

e

Le

SU

CSh

The non-dimensional lift-off height curve (reproduced from [12])

Radiation from jet flameTable 7 Comparison of radiative fraction, χr , of various fuels (reproduced from [30])

Fuel fs χr

Brzustowski

[34]

χr

Burgess and Hertzberg [33]

χr

Tan [35]

χr

Kent [36]

χr

McCaffrey [33]

Hydrogen 1.0 0.2 0.17 - - -

Methane (C1) 0.189 0.2 0.23 0.20 0.19 0.22

Ethylene (C2) 0.170 0.25 0.36 0.26 0.25 0.38

Propane (C3) 0.176 0.30 - 0.32 0.32 0.302

Butane (C4) 0.175 0.30 0.30 0.37 0.37 -

C5 and higher - 0.40 - - - -

Radiation from jet flame

Radiative fraction measured by McCaffrey [33].

Radiative fraction is constant in the buoyancy-controlled regime, but for momentum-controlled jet flames, the radiative fraction decreases until blow-off occurs.

Flame height per nozzle diameter as a function of (reproduced from [30]).

Radiation from jet flames

Effect of crosswind on the radiant energy

Radiation fraction χr decreases with increasing jet velocity

Radiative properties of hydrogen jet fires - (Schefer et al. [20], Molina and Schefer [21] and Schefer et al. [22])

Table 8 Flame conditions (reproduced from [20])

Radiative properties of hydrogen jet fires - (Schefer et al. [20], Molina and Schefer [21] and Schefer et al. [22])

Profiles of radiative heat flux along the centreline of a turbulent, hydrogen-jet flame. Jet diameter is 7.94 mm. Jet orientation is vertical (reproduced from [20]).

Profiles of normalized radiative heat flux along the centreline of a turbulent, hydrogen-jet flame. Jet diameter is 7.94 mm. Jet orientation is vertical (reproduced from [20]).

Radiative properties of hydrogen jet fires - (Schefer et al. [20], Molina and Schefer [21] and Schefer et al. [22])

The radiative fraction for H2 flames is nearly a factor of two lower than non-sooting hydrocarbon flames

Fig. 24 Radiant fraction as a function of flame residence time (reproduced from [20]).

Fig. 25 Variation of radiative fraction (χrad) with the factor for the gas

mixtures in Fig. 24 (reproduced from [21]).

4adpG Ta ××τ

Radiative properties of hydrogen jet fires - (Schefer et al. [20], Molina and Schefer [21] and Schefer et al. [22])

Radiative fraction of large-scale jet flames was smaller than that predicted by the correlation obtained for laboratory-scale flames.

Table 10 Flame width and optical thickness (κ =ap × Wf/2) for the different gas mixtures (reproduced from [21])

Radiative properties of hydrogen jet fires - (Schefer et al. [20], Molina and Schefer [21] and Schefer et al. [22])

Fig. 27 Line-of-sight calculations (RADCAL [40]) of (a) spectral intensity divided by pathlength (ι) and (b) transmittance for flame A of the TNF Sandiadatabase [43-45] (Wf = 0.05 m) and for flames scaled with dj to obtain different values of Wf. Predictions are for x/Lf = 0.7. (reproduced from [21])

Horizontally oriented high pressure hydrogen jet flames

Fig. 29 Hydrogen concentrations at positions: 3, 4, 5, 6, 7, 8, 9 and 10 m downstream from the release point for the 135 barg release from a 3 mm orifice (reproduced from [38])

Fig. 31 Jet flame of 40 MPa high-pressure hydrogen (reproduced from [69]).

Ignition of Hydrogen Leaks

• Astbury and Hawksworth [47]Table 11 Frequency of occurrence of ignition sources (reproduced from [47])

Ignition source Hydrogen incidents Non-hydrogen incidents

Number % Number %

Arson 0 0 37 2.6

Collison 2 2.5 121 8.4

Flame 3 3.7 113 7.9

Hot surface 2 2.5 56 3.9

Electric 2 2.5 114 7.9

Friction spark 2 2.5 33 2.3

Not identified 70 86.3 942 64.5

Non-ignition 0 0 21 1.5

Total 81 100 1437 100

Hydrogen releases

81 incidents all led to ignition

4 cases with delay between release and ignition

11 cases where the sources of ignition

86.3% of incidents source not identified

No-hydrogen releases1.5% did not ignite

64.5% of ignition sources were not identified.

Several mechanisms which could ignite accidentally released hydrogen

• Reverse Joule-Thomson effect• Electrostatic ignition• Spark discharges from isolated conductors• Brush discharges• Corona discharges• Diffusion ignition• Sudden adiabatic compression• Hot surface ignition

Some form of electrostatic charging is a possible mechanism where spontaneous ignition of hydrogen leaks from high pressure occurred at ambient temperature

Several mechanisms which could ignite accidentally released hydrogen

Fig. 32 High speed video frames from large-scale release experiments (reproduced from [46])

Summary• Hydrogen fire burns quickly• Hydrogen flames have low radiant heat• Hydrogen fires are almost invisible • Vertically oriented hydrogen jet fires

– Flame length: Delichatsios’s correlation is valid for vertical turbulent hydrogen jets, both subsonic and choked (validation up to 172 bar)

– Lift-off height: Kalghatgi’s correlation can cover flames from subsonic jets to those from under-expanded sonic jets.

– Radiative properties of hydrogen jet fires • the functional dependence for radiative heat flux established previously for a

range of hydrocarbon flames is also applicable to hydrogen-jet flames.• the radiative fraction of large-scale jet flames is smaller than that predicted

by the correlation obtained for laboratory-scale flames. • the optical thickness increases with the flame size• the radiative fraction for H2 flames is nearly a factor of two lower than non-

sooting hydrocarbon flames

Summary

• Horizontally oriented high pressure hydrogen jet flames – Some tests have been carried out and some are still ongoing.– Little quantified information can be found in the public domain.– It needs to be verified whether the aforementioned findings on vertically

oriented jet flames are applicapable to horizontal flames.

• Ignition of hydrogen leaks– Some form of electrostatic charging is a possible mechanism where

spontaneous ignition of hydrogen leaks from high pressure occurred at ambient temperature.